We rented & drove in Scotland this past summer, and while we've used the coverage provided by our Visa card elsewhere in Europe, we determined we were just stuck with the mandatory "excess" included in the rental price. We did not opt to "buy down" the excess, where for an additional fee, the rental company would've waived more of the deductible than under their regular terms.
You may have noted that, even if you paid the maximum amount for their maximum coverage in case you had an accident or scrapes/dings, they probably exclude tires (tyres), wheels/hubcaps, and side-view mirrors from their coverage, so you'd be liable for those things in any event.
Driving between Inverness and the Isle of Skye, we encountered one guy on the side of the road who'd hit a water-filled pothole that blew out his left tire. We also encountered motorists coming towards us on narrow roads, who failed to stay on their side of the center line (lots of tourists like us from places where you drive on the right side of the road, but some of them had ferried their own cars over, and they were driving with their steering wheel on the left side of the car), so paying for some piece of mind under all those circumstances might not be such a bad deal.
Gas is about $9 a gallon, though, so if you're looking to save money while driving, gas up if you find a place that's even a few pence cheaper per gallon.
